[1] It consists of an outer composite volcano,[5] or stratovolcano,[18] and three younger volcanic vents nested within an amphitheatre that is 2.5 km (1.6 mi) wide and 400 m (1,300 ft) deep.

[5][24][25] The amphitheatre lies at the margin of a rectangular high plateau that is covered by about 2 m (6.6 ft) thick ash,[17][26][27] extending over an area of 50 km2 (19 sq mi).

[54] During the Tertiary, these were overlaid by a total of 300–500 m-thick (980–1,640 ft) deposits from the ignimbritic[j] Capillune, Llallahui and Sencca Formations – all older rock units.

[43][85] The pre-1600 topography of the volcano was described as "a low ridge in the center of a Sierra",[5] and it is possible that a cluster of lava domes existed at the summit before the 1600 eruption which was blown away during the event.

[102] Hot springs occur in the region and some of these have been associated with Huaynaputina;[106] these include Candagua and Palcamayo northeast,[107][108] Agua Blanca and Cerro Reventado southeast from the volcano on the Río Tambo and Ullucan almost due west.

[26] The scale of the eruption and its impact on climate have been determined from historical records, tree ring data, the position of glaciers, the thickness of speleothems[p] and ice, plant flowering times, wine harvests and coral growth.

[73] In the prelude to the eruption, magma moving upwards to the future vents caused earthquakes[119] beginning at a shallow reservoir at a depth of 6 km (3.7 mi);[120] according to the accounts of priests, people in Arequipa fled their houses out of fear that they would collapse.

[126] A sustained eruption column about 34–46 km (21–29 mi) high likely created a mushroom cloud that darkened the sky, obscuring the sun and the stars.

[139] Closer to the vents, inhabitants of the village of Puquina saw large tongues of fire rising into the sky from Huaynaputina before they were enveloped by raining pumice and ash.

[140] It was initially assumed that caldera collapse took place during the 1600 event,[141] as accounts of the eruption stated that the volcano was obliterated to its foundation;[9] later investigation suggested otherwise.

[77] Some collapse structures did nevertheless develop at Huaynaputina, in the form of two not readily recognizable circular areas within the amphitheatre and around the three vents,[142] probably when the magmatic system depressurized during the eruption.

[157] Ash fall from Huaynaputina reached a thickness of 1 cm (0.39 in) within a 95,000 km2 (37,000 sq mi) area of southern Peru, Bolivia and Chile,[152][138] and of over 1 m (3 ft 3 in) closer to the volcano.

[26] The Huaynaputina ash layer has been used as a tephrochronological marker for the region,[5] for example in archeology and in geology, where it was used to date an eruption in the Andagua volcanic field[170][171] and fault movements that could have produced destructive earthquakes.

[172] The ash layer, which may have reached as far as East Rongbuk Glacier at Mount Everest in the Himalaya,[173][174] has also been used as a tephrochronological marker in Greenland and Antarctic ice cores.

[182] The Huayruro Project began in 2015 and aims to rediscover these towns,[183][184] and Calicanto was christened one of the 100 International Union of Geological Sciences heritage sites in 2021.

[182] Estagagache has been deemed the "Pompeii of Peru",[187] and the Peruvian Geological, Mining and Metallurgy Institute has published reports detailing geotourism[r] locations around the volcano.

[191][192] Ash fall was reported in an area of 300,000 km2 (120,000 sq mi) across Peru, Chile and Bolivia, mostly west and south from the volcano, including in La Paz,[18] Cuzco, Camaná, where it was thick enough to cause palm trees to collapse, Potosi, Arica as well as in Lima where it was accompanied by sounds of explosions.

[199] Flooding ensued when volcanic dams in the Río Tambo broke,[99] and debris and lahars reached the Pacific Ocean 120–130 km (75–81 mi) away.

[228] Reportedly, in November 1599 a Jesuit named Alonzo Ruiz had announced in Arequipa that divine punishment would strike the natives for continuing to worship their gods and the Spaniards for promiscuity.

[233] Acid layers in ice cores from Antarctica and Greenland have been attributed to Huaynaputina, and their discovery led to initial discussion about whether the 1600 eruption had major effects on Earth's climate.

[248] Volcanic eruptions alter worldwide climate by injecting ash and gases into the atmosphere, which reduce the amount of sunlight reaching the Earth, often causing cold weather and crop failures.

[245][253] The eruption had a noticeable impact on growth conditions in the Northern Hemisphere, which were the worst of the last 600 years,[5] with summers being on average 0.8 °C (1.4 °F) colder than the mean.

[288] The Huaynaputina eruption was followed by a drought in what today are the Eastern U.S. and may have hindered the establishment of the colony in Jamestown, Virginia, where mortality from malnutrition was high.

[284] The Spanish explorers Sebastián Vizcaíno and Juan de Oñate visited the US west coast and the Colorado River Delta in the years following the Huaynaputina eruption.

The effects of this eruption and the activity of other volcanoes – namely, large scale flooding – might have induced them to believe that California was an island; this later became one of the most well known cartographic misconceptions of history.

[279] Tree ring analysis suggested cooling in Greece,[297] Lapland (Finland),[298] the Pyrenees and central Spain, the Swiss Alps and Switzerland (in 1600) more generally,[299][300][152] where reconstructed winter temperatures were the lowest of 1525–1860.

[296] Anomalous weather conditions relating to the 1600 eruption, possibly under additional influence from reduced solar activity, have been noted in sediment cores from peat bogs in England and Denmark.

[316] Cooling was also noted in tree rings of the Kola Peninsula[298] and ice cores on Novaya Zemlya,[317] where glacier melting rates declined.

[334] Likewise, droughts recorded in cave deposits of southern Thailand have been related to the Huaynaputina eruption and may reflect a typical response of tropical rainfall to volcanic events.

[144] Evacuation of the area directly around the volcano would be difficult owing to the poor state of the roads, and the tephra fallout would impact much of Peru's economy.